Power line carrier frequency telephone system



Oct. 11, 1949.

J. w. EMLING POWER LINE CARRIER FREQUENCY TELEPHONE SYSTEM y Filed oct.11, 194e 2 Shee'cs-Shee'cl l N Q R.

Nb MA Puuh@ .norma u .niv

INVENTOR' J. W. 5ML/Na BV @d @da ATTORNEY Oct. 1l, 1949. .1. w.1EM1 |NGPOWER LINE CARRIER FREQUENCY TELEPHONE SYSTEM Filed oct. 11, 1946 2Sheets-Sheet 2 Arron/vn atented Oct. 11', 1949 POWER LINE CARRIERFREQUENCY TELEPHONE SYSTEM John W. Emling, Morristown, N. J., assignorto Bell Telephone Laboratories, Incorporated, New York, N. Y., acorporation of New York Application October 11, 1946, Serial No. 702,693

6 Claims.

This invention relates to communication systems and particularly totelephone systems including subscriber stations arranged for carriercurrent operation over a ypower transmission line.

Objects of the invention are the provision of carrier telephone serviceover a power transmission line including means for holding theassociated line to the telephone central office on calls betweentelephone stations connected to the power line, the elimination ofmanual control means at these stations heretofore provided for use onlyon such calls, and in general the simpliiication and improvement ofpower line carrier telephone systems.

This invention is a telephone system comprising subscriber stationsarranged for carrier current operation over a power transmission line towhich they are connected, carrier terminal equipment being provided forconnecting the power line to a telephone line terminating in a telephonecentral oflice. Carrier current of one frequency is used fortransmitting over the power line, voice and signal currents to any oneof the stations and carrier current of another frequency is used fortransmitting voice and signal currents from any one of these stations.On a call between stations connected to the power line and utilizing thesame carrier channel over the power line, the carrier transmitting andreceiving frequencies are automatically interchanged at the callingstation and a reverting call signal is transmitted to the carrierterminal to terminate the transmission of carrier current therefrom,this signal resulting from a beat between the carrier currenttransmitted from the carrier terminal equipment and the carrier currenttransmitted from the calling subscriber station.

A clear and complete description of the invention will be facilitated byconsidering a system in which the invention and its features areembodied, such a system being represented schematically in the drawing.The invention is, however, not limited in its application to the systemshown but is generally applicable to telephone systems. The invention isalso applicable to radio transmission and carrier transmission systemsover media other than power transmission lines.

The drawing, which consists of two gures, represents an automatictelephone exchange serving subscriber stations some of which arearranged for carrier operation over a power transmission line.

Fig. 1 indicates a conventional automatic telephone exchange comprisinga subscriber station l0, subscriber lines Il and 2| and line circuits 2l2 and 22, line nder switches LFI and LFZ, selector switches SI and S2,connector switches Ci and C2, a carrier terminal equipment CT individualto the line 2l, and a power transmission line PL to which the carrierterminal equipment CT is coupled by coupling means 44.

Fig. 2 shows one of a plurality of subscriber stations connected to, andarranged for carrier current operation over, the power line PL.

The subscriber station I9 is of the usual type provided for use on linesterminating in an automatic central oilice and includes a dial orimpulse sender (not shown) for use in selectively controlling theswitches through which desired connections are established. The linefinder, selector and connector switches are of the two motionstep-by-step type, each of the line ilnder and selector switches beingrepresented in the drawings by a set of brushes and a single set ofterminals and each of the connector switches being represented by a setof brushes and two sets of terminals. The relays and other apparatusassociated with the line finder, selector and connector switches arerepresented by broken lines. Reference may be had to Automatic Telephonyby Smith and Campbell, 2nd Edition, pages 53 to 65 for a detaileddescription of the structure of two-motion step-by-step switches andtheir operation when used as selectors and connectors. Reference may behad to the patent to R. L. Stokely 1,799,654, granted April 7, 1931, fordisclosure of a line circuit and a line finder of the step-by-step type.The ringing code generator 2l comprises a source of ringing current andmeans for interrupting the ringing supply conductors as required toproduce a plurality of different ringing codes for signaling subscriberstations. Reference may be had to the patent to Duguid No. 2,225,907,granted December 24, 1940, for disclosure of such a ringing codegenerator and to the patent to Koechling No. 2,289,503, granted July 14,1942, for a detailed disclosure of a connector switch arranged for tenparty code ringing.

The carrier terminal equipment CT, power line PL and the carriertelephone subscriber stations of Fig. 2 constitute a power line carriertelephone system similar generally to the system disclosed in thecopending application of R. C. Edson and J. W. Emling, Serial No.653,254, filed March 9, 1946, now U. S. Patent No. 2,481,915, grantedSept. 13, 1949, to which application reference may be had for discussionof the carrier frequencies suitable for use in such systems and for amore complete disclosure of the coupling means between the power lineand the carrier receivers and transmitters and details of the carrierequipment CT and carrier telephone stations. The power line PL is onephase of a power transmission line and serves a plurality of power andlight customers, some of which are equipped for carrier telephoneservice as shown in Fig. 2.

The carrier terminal station or equipment unit CT comprises a hybridcoil or group of transformer windings 39 arranged to transmit signal andvoice currents to and from line 2|. The resistor 3| and condenser 32constitute a network connected to the middle right pair of windings tobalance the line 2|. The upper pair of wind` ings receive voicefrequency currents from a carrier receiver comprising theampliiier-demodulator 50; and the lowest pair of windings transmit voicefrequency currents from line 2| to the v input circuit of a carriertransmitter comprising the oscillator-modulator-amplier 49. The powerline PL is connected through coupling means 44 and Vconductors '43 tothe carrier receiver coinprising receiving band lter 49 and theassociated amplier-demodulator 50, to another carrier receivercomprising receiving band filter 59 and ampliiier-demodulator VV(i0, andto the carrier transmitter comprising transmitting band filter 4| andoscillator-modulator-amplier 40. The amplier-demodulators 59 and and theoscillator-modulator-amplier 40 may be of any known suitable type. Theoscillator-modulator 49 is tuned to transmit through filter 4| carriercurrent of a frequency F! as modulated by either signaling or voicecurrents incoming over line 2|. rThe amplier-demodulator 56 receives,through filter 49 from-the power line PL, carrier current of a frequencyF2 modulated by voice and signal currents from any one of the carriertelephone stations connected to the power line PL. Theamplier-demodulator 69 is tuned by band lter 59 to receive carriercurrent of frequency FI transmitted from oscillator-modulator-amplifier40 and to receive carrier current of a frequency (FI-i-a) where a is alow frequency, for instance, 200 cycles per second, (Fl -i-a) being thefrequency of the carrier which is transmitted from the calling one ofthe carrier tions.

The carrier terminal equipment CT further comprises relay controlled byringing current incoming over line 2| to energize theoscillatormodulator-amplifier and thereby transmit impulses of carriercurrent corresponding to the ringing code received by relay 35 from line2|. The carrier terminal equipment CT further comprises relays 5! and 52controlled by amplifierdemodulator 59 responsive -to signals transmittedover the power line by carrier current of frequency F2 from any of thecarrier stations. The functions of each of the relays is hereinafterdescribed in detail. The sources of energy for operating the relays andfor energizing the ampliers, modulators and demodulators is representedyas being batteries Bl and B2 but may be supplied by rectifying theoutput of a transformer which receives its energy from the power linePL.

Each of the carrier telephone subscriber stations is similar to the oneshown in Fig. 2 and is connected through individual coupling means 59 tothe power line PL. Each station comprises a carrier receiver, a carriertransmitter, a telephone receiver 84 and transmitter 85 combined ahandset which normally rests on the receiver hook 83, a dial 86 forcontrolling automatic switches in the telephone central oiiioe toestablish desired connections, a relay 79 and a ringer 19. The carrierreceiver comprises a receiving 'band filter 'H tuned to pass the carrierfrequencies Fl and (FH-a), a receiving band filter "i2 tuned to pass thecarrier frequency F2 and amplier-demodulator apparatus i3. The carriertransmitter comprises oscillator-modulatoramplier apparatus 88, atransmitting band filter 99 for transmitting voice modulated carriercurrent of frequency (FI-l-a) and also current of frequency F2, afrequency control relay 99, oscillator crystals 9| and 92, and atransmitter control relay 99 for connecting the output conductors offilter A89 to the power line. The transmitting and receiving filterstransmit the voice frequency sidebands in addition to the carrierfrequency. The oscillator-modulator-amplier apparatus, conventionallyillustrated `as .a rectangle 88, includes an oscillator whose frequencyis controlled by the one or .the .other of the crystals 9|, 92 dependingupon the `operated or unoperated condition of relay 90 together withsuitable amplifyingV and modulating means Vwhereby the carrier waves aremodulated -at'times in accordance with voice waves applied throughtransformer 81. The oscillator may be any `suitable and well-knowncrystal controlled oscillator such as found shown in Cadys book onPiezoel ectricity, first edition,

1946, published by McGraw-Hill Book Company, Incorporated, pages 495 and499, Taylor Patent 1,639,817, dated August 23, 1927 or Nicholson Patent2,212,845, dated August 27, 1940. The one or the other of the twocrystals 9| or 92 is switched into circuit to establish the carrierfrequency at-one value or another at different times as hereindescribed. Modulation may be accomplished in any suitable well-knownmanner by means of any one of many well-known modulating circuits ofwhich the following examples are given: Van der Bijl Patent 1,350,752,August 24, 1920 and Helsing Patents 1,343,562, June 15, 1920 and1,560,054, November 11, 1925. Lack Patent 2,218,209, October 10, 1940discloses a suitable combined oscillator and modulator which couldreadily be arranged for shift from one crystal to another Aby relay.9.0. kDevi-ces of this kind are considered to be so well known in theart that they need `no further description. The power supply for theamplifier, modulator and demodulator apparatus includes a rectifier 61rand a transformer 55, connected to the power line either directly orthrough an intermediate transformer. Other transformer windings mayprovide the energy for heating the laments of vacuum tubes and supplywhatever other operating potentials are required. No reverting call keyis provided since the transmission of a reverting call signal isautomatically effected.

Cdl! from station 10 to a carrier telephone station Assume that a callis initiated at station I9, that the line l is connected by a linefinder 'LFI to a selector Si, that thefselector Slis selectivelycontrolled by dial impulses from station I 0 to select a group of trunksand an idle trunk therein associated with a connector Cl, and that theconnector C| is selectively controlled by dia1 impulses from station I9to select the line 2|. Assume further that the line 2| is idle whentested by the connector CI, that a ringing code is selected responsiveto dial impulses from station l0, and that the selected ringing code istransmitted through the brushes of connector CI over the conductors ofline 2| through condenser 33, rectifier 34, and a back contact of relay52 to the winding of relay 35. Relay 35 is thereby operatively energizedduring each ringing interval of the ringing code transmitted over line2|. The operation of relay 35 connects the positive lpole of battery B2tc conductor 39 to energize the oscillator-modulator-amplifier 49,disconnects the lower pair of windings of hybrid coil 39 from the inputconductors of oscillator-modulator-amplier 4U, and connects theseconductors through condensers 3G to the conductors of line 2|; wherebythe ringing current incoming over line 2| modulates the carrier currenttransmitted through lter 4|, conductors 43 and coupler 44 over the powerline PL to each of the carrier telephone stations connected to the powerline.

At each of the carrier telephone subscriber stations the ringing currentmodulated carrier of frequency Fl incoming over power line PL istransmitted through a coupler 69, conductors 1Q, and a normally closedcontact of the springs controlled by receiver hook 83, through receivingband lter '1| to the input circuit of amplifier-demodulator 13. Theoutput circuit of amplifier-demodulator i3 includes resistor 'I4 and thewinding of relay and relay 'i6 is operated responsive to carrier currentincoming over the power line. A condenser i5, connected in parallel withthe winding of relay it, renders this relay somewhat slow in operatingso as to tend to prevent its operation responsive to interferingcurrents such as static. The output conductor of modulator i3 is furtherconnected te condenser 8| thence through the offnormal contacts of dial85 and telephone receiver 84 in parallel with resistor 82. While thetelephone handset is in normal position on hook 93, receiver 84 andresistor 82 are short-circuited at a back contact of the springscontrolled by hook 83. The operation of relay 15 closes a circuit fromthe lower winding of transformer 66 through rectier t?, conductor 68,resister 8i), front contact of relay it, condenser 1l, and through thewindings of ringer i9 to ground at a back contact of the springscontrolled by receiver hook 83. The windings of ringer i9 are therebyenergized while condenser il is charging. Since the incoming carriercurrent of frequency Fl is modulated by the frequency of the ringingcurrent transmitted over line 2| from the central oflice, relay 'i6 isalternately operated and released at the ringing current frequency (forinstance 16 cycles per second) during each ringing interval of theringing co-de. Each time relay 'i9 releases, condenser 'il dischargesthrough resistor 18; and each time relay i6 reoperates, condenser 'Vi ischarged through ringer i9. Thus, the ringer 19 at each of the carriertelephone subscriber stations associated with power line PL is actuatedin accordance with the ringing code of the particular station which isbeing called.

When the handset is removed from the hook 83 at the called station, thecircuit through ringer I9 is opened, the short circuit across thereceiver 84 is opened, and the positive potential supply conductor 68 isconnected to the winding of. relay 93 and to theoscillator-modulator-amplifier 88, operating relay 93 and energizing theamplifiermodulator 38. Relay 93 is somewhat slow in operating to allowtime for relay 98 to operate if relay T6 is operated at the time thehandset is removed, as is the case at the calling station on a revertingcall as hereinafter described. However, on the call now being described,relay 'i6 is not operated at the called station when the handset isremoved because at this time there is no carrier current of frequency FZbeing transmitted over the power line, so that relay 99 is not operated;and the operation of relay 93 disconnects the winding of relay 99 fromthe armature of relay 16, thereby to prevent the operation of relay 90on this call from station l. Since relay 99 is normal, the oscillatorcrystal 92 is included in the oscillating circuit ofoscillator-modulator 88, whereby carrier current of frequency F2 isgenerated and transmitted through filter B9, a front contact of relay93, conductors 'i9 and coupler 59, over Ipower line PL to each 0f theother carrier stations and to the carrier terminal equipment CT. Thecarrier current of frequency F2 does not reach the demodulator i3 ateach of the other carrier subscriber stations because, at each of thesestations, the input circuit of filter i2 is open as long as the handsetis in normal position on hook 83. But at the carrier terminal equipmentCT, the carrier current of frequency F2 passes through coupler 44,conductors 49 and receiving band lter 49 to the input circuit ofarnpiier-demodulator 59 causing the operative energization of thewinding of relay 5i. The operation of relay 5| closes a circuit foroperating slowto-release relay 52 and closes a bridge, consisting of theleft middle pair of windings of hybrid coil 3D, a cross line 2| to tri-pthe ringing in the connector Cl The opening of the ringing circuit andclosing of the talking connection through connector CI is effected inusual and well-known manner. The operation of relay 52 opens the circuitconnection between the winding of relax7 35 and line 2|, and connectsthe positive pole of battery B2 to the amplifier-demodulator 59 and tothe oscillator-modulator-amplier 4d. The energization of theamplifier-demodulator @d performs no useful function on this call; butthe energization of oscillator-modulator-amplifier 49 effects thetransmission of carrier current of frequency Fi through filter 4| overthe power line PL to each of the carrier stations, causing theenergization of relay TE at each of these stations. The voice currentsincoming over line 2| from the calling station |9 are transmittedthrough hybrid coil 3! to the input circuit 0f modulator-- amplifier 40to modulate the carrier current of frequency F| transmitted over thepower line to the carrier stations. At the called station, the voicecurrent output of ampliiier-demodulator 'i3 is transmitted through thereceiver 84, but at the other carrier stations, the receiver isshort-circuited at a contact of the receiver hook springs. Thetransmitter of the handset of the called station generates voicecurrents which are transmitted through coil 8'! to the input circuit ofmodulator-ampliiier 88 to modulate the carrier current of frequency F2which is being transmitted over the power line. The Voice modulatedcarrier current of frequency F2 passes through filter 49 to demodulator50; and the voice currents are transmitted through hybrid coil 3|! overline 2|l through the` connector; selector and; line;r findenI switches,.over line ll.- tothe calling..V stationz lef, Two-way conversationf nowtakes..place,=. over. the? established connection: including powerf linePLy between' the calling and called station.

When the: handset is. replaced` on hoolr-l 83;' of.' the called station,receiver 84` isV short-circuited,v relay Sfis released-,and theoscillator-modulator@ amplifier is deenergized;Y terminating.;- the"transmission' of carrier current. of' frequency: E over.- the power lineand-thereby causing the `.suc-- cessive release of relays 51|y and52of=lthe carrierf terminal equipment CT. The=releaseofirelaysilopensfthe bridge across line-2l thereby'releasingS thez answering.supervisory relay: (not shown) .y in; the. connector CI'- Therelease ofrelay fdisconnects battery BZ'from oscillator-modulatoramplier il@ toterminate the transmission-.ofthe carrier current of frequency FIV overthe power line, thereby releasingY relay 'Hi at. each` ofvthe carrierstations. YWhen the subscriber at ther callingfstation la releases theconnection; the Iconnector.V C1, selector Sl` and line iinderLll-A arerestored to. normall in usual and' Welllknown.

manner.

Call from carrier. telephone station Assume next thatv the handsety isremoved'from; hook Bti-at' one of: the carri-er telephonevstationsconnected to power` line PL to,y initiate a, calL. f-

Relay 93` isthereby operated;. relay remains;

Vnormal andthe oscillator-n'lodulator-ampiiier Sais-energized@ transmitcarrier current of frequency F2.- over power line PLcausingthe-sucscessiveoperation of relays 5l and52zof carriers` Yterminal` equipment CT. At eachV of the; other carrier subscriberstations, the carrier'currentofi frequency F2 transmitted over line.; PLfrom the: calling carrier station is.v ineffective. to. operate: relay'F6 because the input conductorsf'leare nor--` many-'connected acrossthefinput'v conductors. of; lter 1 l' and are not normally vconnectedacross the..- input conductors of ilter l2. The operationA of; relay 5l-of-.carrier equipment CT closes .the bridge'i across line 2l wherebythe line relayl (notshown)v of line circuit 22 is operated to startlanidle line. finder such as LFZftoconnect with the callingiline; and line2| isthus connected. through the brushes of line finder LFE' to theassociated sel'eetorSZ'.l The operation ofI relay 52 opensthecircuitconnecton` between relay 35-and line.2l1andconnects. batteryB2` to oscillatoremodulator-amplifier' 60S and to-ampliier-demcdulatorThe energizaftion ofoscillator-modulaton-amplier: 40:' effects thetransmissiony of: carrier currentiof'. frequency FIy over power line;PL. Dial tone isz transmitted from selector S2 :over line. 2 lthroughhybridlcoil :3m totheinmit of.- modulator All to modulate-'thecan rier currentV ofl frequency FI being t1'.a1'1s1nitted:v over.theepowerfline; to; the?carriertelephonesta-A tions. The carriercurrentof frequency Flfeifectsv the operatonfofrelay 'l at eaoh ofthefcarriersta-y tions; and, at'. the callingr carrier sta-tion., thefdialitonele/transmitted through'the receiver M: toindicate that the dialingof the number of; the telephone station with whichconrrection: isdesired. maybe starte-d..r The calling party theres upon. dials. thedirectory number of' the*v called: station. Whenthe dial is moved offnormal, the;

receiver 84 is disconnectedffrom amplier.demod ulator 1'3`; and, as thedial returnsto..normal, the.. irnpulsecont'act's ofthe dial modulatethecarrierl current` of frequency F2 being transmitted from.the*oscillator-m'o'dulatonamplier 8B: Each dial'.

. amplifier Lll.

i over'A the: powcn1 line causes.: thee releases and; rf operation of'.relay 5l.: Each. release; and'.y reoperu ation ofirelaysl; responsive-toidialimpulses, opens.Y andv closes the? bridge` across line 2 I therebyref*- peating: the-dial impulses to selectively' control:

the operation; ofL selector.vr SZvand arconnect'or-GL to@ extend. theiconnectionl to the called line;` and t0; select the; ringing; coderequired#V for'signaling-1l the`l called station:- Since-relay52iss1ow"t0.2re-

. lease, itf remains operatedif during. the responseroft;

relay: toi dial impulses.

Assume first: thatzthe called` station is'not'; onef of: the carrierstations connected? to thef power.- lineiPL. Whenctlie oallfis-vanswered, the'oonneef-v tor C2 opensstherv ringing; circuit andiclosesthei talking circuit inusual andxwell-known manners; and :talkingcurrentspzare transmitted throughhy.-y brid coil.: 3l=andoverthe powerlinef inibothzdirecf tionsa. The; connection is undert the control: offthecalling carrier telephone station; and; Wheni the handset. ofl'thisfstation isrestored` to. normal?. conditionon the; hookf: 63',relay 93., is releasedand; the oscil1:ttor-modulator-arnpliiier- 88-is'- deenergized; terminating:thetransmission of carrier our;A

. rent offrequencyF-Z andgtl'ierebyrjcausing the suc;

cessi-veerelease; of relays: 5l'V and 52 offth'emarreit'Y terminalequipmentv CT. The release of` rela-w51: opensfithebridge acrosszline-Zland the releaseeof relays 52: deenengizes the oscillator-modulator Theopening of the bridge across; line 2.12 causesthe release; of the.V linerelay (not. shown) of the connector C2- and'returncof'. the: selector;S2.- and line. indeifLFZ to, normal in usuali manner. The connector C2vis also restoredto:`

f normalin usual manner as sooncas both the calleA ing and called;-stations have:` released. the conf-- nection.,

Assum'elnext. that the. call initiated` at azcarriei: telephone..Vsubscriber stationv as above described; is. a. revertingcall;` that isa; callto anothersubscriber; stationv connected. to; the power'V line:PIs.; Invsuch: a. case; theconnecter GZ'nds the called:Y line. 2l'.busy' wherritrnakes.;the.l busy test',Y and; a busy. tonef is.:thereupon` transmitted" from. the? connecton throughr selector. S2, and"line LE2,xover line 2|, through hybri'd'tcoil 30;.modu-- lator-amplierr;ltei. lgpower line PL, filter* Tlf, ampli'eredenrodulator '131. andcondenser.` 8-12 til-telephone.receiverrBllatithecalling.station; The?connector G2 is arranged to make ay reverting. calltest and; ifithe;call. is alreverting call, the?. line: inden LEZA and selector.K S2.are; restored; tou normal when the calling: subscriber-hangs; ups., but:the connector G remains connected'. to` the'.Y

` line 2lbto.ringi'thefcalledlstationz. Referencefmay."

behad: to the' patent to I-I. Hovland. 1,849,0{l7`,` granted March I5,1932; for; disclosure ot a. conm neotorrarrangedtoxmake;` theIaforementioned; reil verting call test; Upon. hearing: thefbusy tone;-transmittedifro'm connector C2, the callingsubscriberhangs the-handsetiomthe'hook. Relay e3` releases andi the:v oscillator-modulatoreampligfer- 88 eis: deenergized to terminate the; transmisesion.` of; carriercurrent offrequency-F2 over-'ther l power line and thus release relay5I; of. thevcar' rierl terminal equipmentzCT. 'Ihe'release'rofy rea lay5l. opensv the bridge across the conductorsvof" linefl-J; theeline nderfLFZ and selector- S2 are.: restored to .normaL and the connector G2 isoperi-A ated., to. closey thefringng.v cirouit.- and apply ther; ringingcode of the called station.- tothe conclue--n tors.of.1ine' 2l...Tfherelease of relayl alsocauses the release of relay 5.22, but.` theenergizationf of. the ringing; relay responsive to, ringing, our--impulse thus transmitted fromthecalling station rent received over line'2l' effects the energi'zation.

of the oscillator-modulator-amplier 4D to transmit ringing currentmodulated carrier current of frequency FI over the power line PL. Ateach of the carrier telephone subscriber stations the relay 'i6 respondsto the ringing current and causes actuation of the ringer 19 accordingto the ringing code of the called station. When the called subscriberanswers by removing the handset from hook 83, relay 93 is operated andthe amplifiermodulator 88 is energized to transmit carrier current overthe power line. Since at this time there is no carrier current offrequency F2 being received over the power line, relay 16 is notenergized at the called station and relay 90 does not operate. Withrelay 90 normal, oscillator crystal 92 is connected in the oscillatorcircuit of modulator 88 so that the carrier current transmittedtherefrom is of frequency F2; and with relay 93 operated and relay 90normal, the input circuit of lter ll is closed and the input circuit offilter 'l2 is open, so that the amplier-demodulator 'i3 is tuned toreceive carrier current of frequency IFl and frequency (F I-l-a). Thetransmission of carrier current of frequency F2 over the power line PLfrom the called station eifects the reoperation of relays and 52 of thecarrier terminal equipment CT. The operation of relay 5l closes thebridge across the conductors of line 2l to trip ringing and to operatethe answering supervisory relay (not shown) of the connector C2 in usualmanner. The operation of relay 52 connects the positive pole of batteryB2 to energize the oscillator-modulator-amplifier All and also toenergize the amplier-demodulator 60. The energization of theoscillator-modulator-amplifier 4B eects the transmission of carriercurrent of frequency Fl over the power line to energize relay 'le ateach of the carrier telephone stations.

When the operation of the ringers at the carrier telephone stationsceases due to answer of the call at the called station, the callingparty removes the handset from receiver hook 83 thereby opening theconnection between input conductors 'lli and the input conductors offilter 'H and connecting input conductors 1D to the input v.

conductors of lter 12. Since, at this time, carrier current of frequencyF2 is being transmitted over power line PL from the called station,relay 15 is operated at the calling station and removal of the receivercloses a circuit through the winding of relay eil; which circuitincludes the outer right back contact of relay 93, a normally opencontact of the springs controlled by hook 83, front contact of relay i6,resistor 85, conductor .68, rectifier 5l and the lower winding oftransformer 55. Relay 9G is thus operatively energized before theslow-to-operate relay 93 operates, and relay 99 locks independent ofrelay 93. The operation of relay 913 includes the oscillator crystal Siinstead of crystal 92 in the oscillating circuit of modulator 88 wherebycarrier current of frequency (Fl-l-a) is transmitted from the callingstation instead of carrier current of frequency F2. With both of relays90 and 93 operated at the calline station, the input circuit of lter l2is closed and the input circuit of filter 'il is open; so that voicemodulated carrier current of frequency F2 incoming from the calledstation is transmitted to the receiver 84. The carrier current offrequency (FH-a) transmitted over the power line from the callingstation passes through filter 59 at the carrier terminal toamplifierdemodulator il() thus causing the operation of relay 6 l. Thecarrier current of frequency (Fl -l-a) incoming over the power linefrom' the calling Cil station and the carrier current of frequency Fltransmitted over the power line from oscillatormodulator-amplier 40produces a beat of a cycles per second-for instance, 200 cycles persecond. Relay 62 is tuned to operatively respond to energization of itsright winding by current of the beat frequency. When operated, relay 62is held operated by its left winding under the control of relay 5i Theoperation of relay 62 disconnects battery B2 fromoscillator-modulator-amplifier Ml to terminate the transmission ofcarrier current of frequency FI therefrom while conversation is takingplace between the calling and called stations. It is to be noted thatthis arrangement has the advantage that if the receiver is removed atsome carrier station other than the calling or called carrier stationwhile a call other than a reverting call is in progress, thetransmission of carrier current of frequency Fl from the carrierterminal equipment will be resumed as soon as the receiver is replacedat the interfering station.

If the calling subscriber hangs up rst, relay @t releases andoscillator-modulator-amplifier 88 is deenergized, whereby thetransmission of carrier current of frequency (Fl-l-a) from the callingstation is terminated, causing the release of relay 6i and 52 of carrierterminal equipment CT and the release of relay 'I6 at each of thecarrier stations not involved in the call described. The release ofrelay 6| connects battery B2 to oscillator-modulator-amplier il wherebycarrier current of frequency Fi is now transmitted from the carrierterminal equipment CT over the power line causing the reoperation ofrelay 'i6 at each of the carrier stations. Relay 5i is held operated bythe carrier current of frequency F2 being transmitted over the powerline from the called subscriber station. When the called subscriberdisconnects, the oscillatormodulator-amplifier 88 thereat is deenergizedterminating the transmission carrier current of frequency F2 over thepower line, releasing relays 5I and 52 of the carrier terminal equipmentCT. The release of relay 5i opens the bridge across the conductors ofline 2l thereby causing the release of the connector C2 and the releaseof relay 52 again connects the winding of relay 35 across line 2i forresponse to ringing current on a succeeding call.

If the called subscriber hangs up rst, the transmission of carriercurrent of frequency F2 therefrom is terminated causing the release ofrelay 5l of the carrier terminal equipment CT and causing the release ofrelays 'l5 and Se at the calling subscribers station. The release ofrelay 90 reconnects the oscillating crystal 92 in the oscillatingcircuit of modulator 88 in place of the crystal 9i whereby thetransmission of carrier current of frequency (Fl-l-a.) is terminated andthe transmission of carrier current of frequency F2 is initiated at thecalling subscribers station. Relay 5i of the carrier terminal equipmentCT is thus reoperated before the slow-torelease relay 52 has released soas to hold the central office connection until both of the subscribershave disconnected. When the calling subscriber hangs up, relay 90releases and the oscillator-modulator-amplifier 33 is deenergized at thecalling station.

If the called subscriber fails to answer on a reverting call, the ringerat each of the carrier stations is actuated by the ringing current untilthe receiver is removed from the switch-hook at some one of thestations. When the receiver is replaced at the subscriber station, thecarrier l-ll iterminal-.equipmentfl` is restored to normal as `Wellzastlfiecarrier equipment at-the answering sstation. f

.1.;In a telephonefsystemaftelephone line, :a

vpower utransmission .line, sa carrier :terminal sta .tioninterconnecting :said lines, a plurality =oi :telephone vsubscriberstations A connected to said powerd-ine,- :said .carri-er t'ern'linallstation compris- .ing a-narrierf currenttransmitter.;for-:transmitting.over-.the pewerflinefea-rrierseurrent Oia fre- -guencyf a :firstcarrier freceiver for receiving over thegpowerfline carrierfcurrent ofassecond fre ...quency a-ndfasecondearrier receiver for receivfing overthe'` power -lineiearrier current of said iirstf-requencyand carriercurrent of a third ifreguency, each of fsaidfsubscriber stationsVcornprising :carrier current transmitting and yreceiv-:ingfmeansarranged to selectivelytransmit over` fthelpovverrlineearriercurrentccf :said second or said third frequencies and ,-to .receivevoverthe :poweriline `carrier.current of=a1l offsaid .carrier frequencies.means .at `said carrier terminal sta- .tion forl energizing l the.transmitter there- .atresponsive'tocarrier currentof said second ire-'gquency .incoming-over said power,line,.andmeans fat vsaid carrier-terrninal .station =operatively reesponsive `,to the beatbetwseen Nthecarriere. current of said irst frequency ,transmittedxby the .carr-iertransmitter at said ..carrier .terminal station rand PtheIcarriercurrent .of said .third frequency .received-rover `tl1e.,povverllinefrom the calling stationon a .reverting-.call for disabling thecar- .rientransmitter at .the carrier terminal station.

`In .a telephone system, a telephone line, a lnewer transmission .linea.carrier .ternnnal Station,interconnectingisaidlines,,aplurality oftele- .plionesubscriber stationsonnected to said power iine,:saidcarrier `terminal.statien 4comprislng a normally deenergized .carriertransmitter tuned to 'transmit .carrier .current of .a .rst i'irequency,.eacliof said,subscriberstationsicomprising a carrier transmitterandmeansior tuning the transmitter to transmit .carrier current of a.second frecuency .on .calls rover Asaid Ltelephone ..1ine, .to'transmit .carrier .current .of said second .frequency Aon .incomingreverting, calls -and to i transmit. carr-iencurrent. of a .third.frequency on outgoing .reverting calls, each of saidsubscriberfstations.fur- .tner comprising acarrienreceiver .tunedattimes to receivecarrier current of..said .rst and .third 'frequenciesand .tuned attimes to i receive .carrier 'currentioffsaid second,.freguencv. saidicarrier .ter-

minal fstation .further `comprising .a `i'lrst carrier Areceiver .tunedto `receivecarriercurrentof said second frequency, means `operated.responsive .to the ,receipt .of carrier `.current .of said; secondfrequency by saidfirstcarrierreceiverifor Aenergizing the carrierwtra-nsmiter .atsaidarrier Vternfiin-al statien, a second.carrier.Lreceiver. tuned to receive carrier current ofsaidifirst:and-thirdifrequencies, means .tuned teoperatively 4respend tc.the .beat between said-.first and thirdfrequencies when .carrier icurrent .of both fsaid and vtlnird frequencies being received-*bv said.seconde. rier receiver. the .operation ef. .said lastenamed meansbeing-effe tive yto deenergize.tne..carrier transmitter: -atfsaidcarrier terminal station .to prevent the energization of saidtransmitter as :long `4as carrier current of Asaid-.tl'iirdvfreguency'is being lreceived fby vsaid-second:carrier receiver.

3. In sa telephone system, :a rtelep'honelina a :newer transmissionline, .a;carrier terminal station interconnectingisaidlinesforztelephonecommunication, and fa A'plurality of telephone Ysuie-..sc-ritienstationsF conneetedfto saidtpowerilineseaeh fof f saidsubscriber stations .comprising za carrier acurrent .transmitter to,selectively transmit over ,said power =line voi'cesfrequency .modulatedcarrier Vcurrent --ef either -one of two -.earrier fre-..quencies-said'terminal station. comprising a .-carercurrentetransmitteretunedtto transmit carrier veurrentf'ofiathird-carrier frequency, arst-car- .rierfcurrentreceiver V.tuned to.receive voice ,fre-

.quencyfmodulatedlcarrierxxurrent of a rst.one tof :said itu/.o carrierfrequencies, .-.a :second-carrier .receiver :fatesaid f terminal:station tuned *.to -.rec.eive .carrier .current 'of Ltherotl-ier of.said :two :gire- Vtqiiencies :and ncarrier current :of .said vthird:frerguency, means controlled by fsaid 'rst .carrier .receiver for:energizingitheyz carrier .current-transnnitter-fat-saidfterminalstatien, and means con- :trol-led by said fsecpnd carrierreceiver vat fsaid `:terminal station treesponsive to ,simultaneously-ceivi-ng .carrier .current tof saidother of saidtwo frequencies andcarrier currentfef said-.thirdcarfrierefrequencyffon deenergizingfthe.carrier .transamitterna-t.:saidterminal station fandifor'maintain-Iingsaidideenergizatien:aslong .asf'ca-rrier current of .sa-id other of.ssaid :two `.frequencies -is fbeing ransmittedover fsai'd power eline,veach 'of :said subscriber stations further comprising -fa :carrierlcurrent .receiver selectively tunable ato :receive .carrier` cur-rent.of i .said firstone fof :said :two frequencies er lto receive carrierscuri-ent fof -said etnerfof said f two .frequencies and carrier currentof :said third frequency, and means feiect-ve '-at ltimes Anesponsive to-earr-ier :current .of :sa-id rst .one cf v"said t two .freqnenciesineoming Iover ysai-d ypervver :line i-for Atuning .the :carriertransmitter rtnereat ,to transmitfcarrierfeurrentfof saidf'other lffsaiditvro frequencies and fortuningfsaidcarrier ireceiver :to :receive:carrier :current of said rst onefzofsaiditwmfrequencies.

4. .Ine vtelephone :system zcomprising Ca dial subscriber itelennonefiine, .fa power transm-ission .line,fa @carrier currentterminalzeguipmentzinterconnecting said zlines, .and a :.pluralityfeffcarrier .telcel-iene ,subscribe-r stations leonnectei to fsaid verline.,4 a carrier :current ftransmitter :in Asaid .rieriterminaliequipment fcritransmitting -carfiere current of afrstffreguency :over -rsai-d ,fpower line,carrierecurrent=transmittingameans k'at each .ef carrier stations for:transmitting carrier currentlnf n :seeondrfreguencylor carrier'xurrentci e.itnirdifrequencyfever said :power yline, a first .carrierfreeeiving means :in :said-:carrier `terminal equipment tun-editereceive .carrier current nf fs aid .frequency :from esaidzpewer line,iarsecond .carrier receiving .means in :said carrier terminal equipmentztuned 'to zreceive fc-arrier cnrrmt fof said instand thirdffreguenc-iesemeans vresponsive .te current `ofy said vsecond frequencyringever-saidipowerilineeto saidrst #carrier receiving meansfornenergizing theccarrier .current .transmitter of `said `carrier1terminal 'equipment and :se-id l:second carrier `:receiving rmeans,'control means-tuned to the beatsfrecluenc-ybetween said rstiend-it-hird frequencies for :deenerg'izing fthe .carrier -current`.transmitter said -equipment and-.means for :maintaining thetdeenergizationf of .the carrierv :currenttransmitter;ofzisaidfequipment.as -iongf'a's carrierzcurrent LOfzSadIthrd 'frequency isfleeing-,received byfsaid iseconrlscarri'er receiving means.

f5. 'In La =.telephone `system :comprising a dial subscriber telephoneline, La #power transmission line, carrier lcurrentiterminalequipmentinterconnecting said inea-antifa fpluralityof carrier iteiephonefsubscriber stations Jconnected Lto '-said 13 power line, a carriercurrent transmitter in said carrier terminal equipment for transmittingcarrier current of a rst frequency over said power line, carrier currenttransmitting means at each of said carrier stations for transmittingcarrier current of a second frequency or carrier current of a thirdfrequency over said power line, a rst carrier receiving means in saidcarrier terminal equipment tuned to receive carrier current of saidsecond frequency from said power line, a second carrier receiving meansin said carrier terminal equipment tuned to receive carrier current ofsaid first and third frequencies, means responsive to carrier current ofsaid second frequency incoming over said power line to said firstcarrier receiving means for energizing the carrier current transmitterof said carrier terminal equipment and said second carrier receivingmeans, control means tuned to the beat frequency between said rst andthird frequencies fr deener- H gizing the carrier current transmitter ofsaid equipment, means for maintaining the deenergization of the carriercurrent transmitter of said equipment as long as carrier current of saidthird frequency is being received by said second carrier receivingmeans, and carrier current receivingr means at each of said carriersubscriber stations normally tuned to receive carrier current of saidfirst and third frequencies and tunable to receive carrier current ofsaid second frequency.

6. In a telephone system comprising a dial subscriber telephone line, apower transmission line, a carrier current terminal equipment interconnecting said lines, and a plurality of carrier telephone subscriberstations connected to said power line, a carrier current transmitter insaid carrier terminal equipment for transmitting carrier current of arst frequency over said power line, carrier current transmitting meansat each of said carrier stations for transmitting carrier current of asecond frequency or carrier current of a third frequency over said powerline, a rst carrier receiving means in said carrier terminal equipmenttuned to receive carrier current of said second frequency from saidpower line, a second carrier receiving means in said carrier terminalequipment tuned to receive carrier current of said rst and thirdfrequencies, means responsive to carrier current of said secondfrequency incoming over said power line to said first carrier receivingmeans for energizing the carrier current transmitter of said carrierterminal equipment and said second carrier receiving means, controlmeans tuned to the beat frequency between said rst and third frequenciesfor deenergizing the carrier current transmitter of said equipment,means for maintaining the deenergization of the carrier currenttransmitter of Said equipment as long as carrier current of said thirdfrequency is being received by said second carrier receiving means,carrier current receiving means at each of said carrier subscriberstations normally tuned to receive carrier current of said first andthird frequencies and tunable to receive carrier current of said secondfrequency, and means at each of said carrier subscriber stationseffective at times responsive to carrier current of said secondfrequency incoming over, said power line for tuning the carrier currenttransmitting means thereat to transmit carrier current of said thirdfrequency and for tuning the carrier current receiving means thereat toreceive carrier current of said second frequency incoming over saidpower line.

JOHN W. EMLING.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,773,613 Clark Aug. 19, 19302,289,048 Sandalls, Jr July 7, 1942 2,294,129 Purrington Aug. 25, 19422,294,905 Honaman Sept. 8, 1942

